As indicated in U.S. Pat. No. 4,078,286, in a typical process for electrophotographic duplication, a light image of an original to be copied is recorded in the form of a latent electrostatic image upon a photosensitive member, and the latent image is subsequently rendered visible by the application of electroscopic particles, which are commonly referred to as toner. The visible toner image is then in a loose powdered form and it can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support, which may be the photosensitive member itself or another support such as a sheet of plain paper. The present invention relates to the fusing of the toner image upon a support.
In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
The use of thermal energy for fixing toner images onto a support member is well known. Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a flat or curved plate member in pressure contact with a roll; a belt member in pressure contact with a roll; and the like. Heat may be applied by heating one or both of the rolls, plate members, or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art and they can be adjusted to suit particular machines or process conditions.
During the operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate, or belt members. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the materials being copied there. The so called "hot offset" occurs when the temperature of the toner is raised to a point where the toner particles liquify and a splitting of the molten toner takes place during the fusing operation. "Cold offset" may be caused, even at the temperatures below the molten point of the toner, by such factors as imperfections in the surface of the fusing members; by the toner particles being insufficiently adhering to the support; by electrostatic forces which may be present; etc.
Another problem frequently encountered in fusing with a heated member is that the substrate, e.g. a sheet of paper, on which the toner image is fused may curl and/or adhere to the heated fuser. Such adhering paper will tend to wrap itself around the fuser and thus prevent the fuser from performing its intended operations in subsequent copying cycles. Such adhering paper must be generally removed by hand, resulting in much manual labor and machine downtime.
Another feature common to most of the prior art fusing members is that the source of the heat energy for the fusing operation is generally in the form of a quartz lamp positioned in the core of a fuser roll. In such a configuration, the heat must be conducted from the core of the fuser member, through the various layers of materials comprising the fuser member, to the surface of the fuser member for transfer to the toner image and its substrate. To obtain the proper fusing temperature at the surface of such a fusing member, it can be readily appreciated that the temperatures at the various layers or points within the fuser member must be substantially higher. Moreover, since heat must be transmitted from the source in the core of the fuser member to its surface, it takes an appreciable amount of time before the surface of the fusing member is warmed up to the fusing temperature and thus ready for operations. This delay in readiness of the machine to fuse toner images, or the warmup time, is accentuated by the fact that such fuser members are generally made of elastomeric or other polymeric materials which are generally poor conductors of heat.
In U.S. Pat. No. 3,452,181, here is disclosed a roll fusing device which is heated by both an internal heating element and an external auxiliary heating element. The fusing drum of this patent is made of a glass or quartz sleeve having a transparent silicone varnish layer thereon and offset-preventing silicone oil is applied to the surface of the silicone varnish layer.
U.S. Pat. No. 3,498,596 discloses a heat fixing apparatus in which the pressure roll is made of a metallic cylinder having a coating of Teflon thereon, and in which the heated roll is made of a metallic cylinder having a heat insulating silicone rubber blanket thereon. The silicone rubber blanket has a thin coating of a reflective release agent applied thereto.
In U.S. Pat. No. 3,912,901, there is disclosed a PFA Teflon sleeved pressure roll. The pressure roll is made of a thick elastomeric layer of heatresistant silicone rubber, with a thin sleeve of a high flex life fluorinated ethylene propylene (FEP) sleeve thereon. The fuser roll of this patent is made of a rigid cylinder coated with a fluorocarbon polymer layer (such as tetrafluoroethylene).
U.S. Pat. No. 3,967,042 discloses a fuser blanket for use in an internally heated fusing roller, which is made of a heat conductive backing, a fluorinated elastomer layer and a thin silicone elastomer overlayer.
In U.S. Pat. No. 4,071,735, there is disclosed an externally heated roll fuser, in which the heating element heats the fuser roll at the same time preheats the toner image to be fused. The fuser roll of this patent is made of a metallic core with a layer of heat insulating silicone rubber thereon.
U.S. Pat. No. 4,083,092 discloses a sleeved organic rubber pressure roll, which is made of a thick layer of a resilient organic rubber on a metallic core. The thick organic rubber layer has an outer sleeve of Teflon or similar material to prevent the oxidation degradation of the thick organic rubber layer. The fuser roll of this patent is made of a thick metallic core with a layer of Teflon or similar material on its surface.
U.S. Pat. No. 4,149,797 discloses a sleeved organic rubber pressure roll which, for the purposes of the present invention, is essentially similar to that disclosed in U.S. Pat. No. 4,083,092 mentioned above.
It is an object of the present invention to provide an improved externally heated fuser roll.
It is another object of the present invention to provide an externally heated fuser roll which is energy efficient, produces copies of high quality, as well as possessing superior release properties.
Other objects of the invention can be gathered from the following detailed disclosure.